1. Field of the Invention
The present invention broadly relates to composite structure fuel tanks, especially for aircraft, and deals more particularly with a laminate edge construction that reduces the possibility of fuel combustion due to an electric charge.
2. Description of the Related Art
Federal regulations are imposing increasingly stringent standards for fuel tanks used on commercial aircraft. These regulations are directed toward reducing the possibility that fuel or fuel vapors in or around the tank might be ignited by ignition sources such as an electrical charge or spark produced by direct lightning strikes or other electrical currents reaching the fuel due to catastrophic structural failures.
Various solutions have been proposed for mitigating the risk of fuel tank ignition, but each has limitations and none has been entirely successful. For example, bladders have been employed to isolate fuel within tanks from the effects of an ignition event caused by failure of the fuel tank structure, however this solution requires relatively complicated tank geometry and is inefficient when applied to commercial transport aircraft. Another solution involves of the use of a completely metallic internal structure for the fuel tank, but this approach is both inefficient from a strength and stiffness-to-weight standpoint, and is expensive relative to other potential solutions.
Composite structure fuel tanks are commonly used in commercial transport aircraft but these tanks also require protection against potential ignition of fuel. Composite structure fuel tanks are typically manufactured from structural members comprising carbon fiber reinforced polymer laminates. The laminate members must be trimmed to size, typically using a diamond saw or water jet cutter. Prior to trimming, the carbon fibers are sealed by the epoxy resin within the laminate, but cutting of the laminate during the edge trimming process results in the carbon fibers being exposed at the cut edges.
Some of these cut edges on the laminated structural members may be exposed to the interior of the fuel tank or to fuel lines where small amounts of fuel or fuel vapor may be present. When subjected to a high energy electrical charge such as that resulting from a lightning strike, and result in sparking between the fibers, or the release of highly energized particles which have the potential to ignite fuel. However, in order to partially mitigate this risk, polysulfide-based sealants have been applied to the cut laminate edges. The application of such sealants requires experienced personnel and is labor intensive in terms of both the sealant application and subsequent in-service inspection. Moreover, these sealants must be applied after the fabrication of the fuel tank structure is completed since the sealant layers are relatively fragile and thus subject to damage caused by handling or assembly of tank substructures. Another drawback of polysulfide-based sealants is that failures in the sealant coating are difficult, or sometimes impossible to inspect. This detection problem is due in part, to the fact that polysulfide-based sealants are opaque, and thus visually mask underlying bonding defects or defects in the composite structure. As a result, failure of polysulfide-based sealants can result in latent defects during in-service maintenance and inspections.
Another potential solution to the problem of reducing the risk of fuel tank ignition involves designing composite fuel tanks without the use of internal cut laminate edges, however this approach generally results in highly inefficient fuel tank designs from a fuel capacity to weight standpoint.
Accordingly, there is a need for a composite structure fuel tank in which cut edges of the laminate are permanently sealed, and both mechanically isolated and electrically insulated from fuel within the tank. The present invention is directed towards satisfying this need.